As a means for improving the resolution of a phase shift mask used in photolithography, a halftone phase shift mask is known, which is composed of regions which transmit light and regions which semi-transmit light. As a typical example of the halftone phase shift mask, known is a halftone phase shift mask having a transmittance of 6% in which a MoSi layer is used as a light semitransmissive layer.
When the pitch of interconnects to be formed in a wafer is made fine, a light semitransmissive layer which constitutes regions which should semi-transmit light in a halftone phase shift mask is required to be a layer having properties of being smaller in EMF bias and OPC bias and larger in exposure latitude and depth of focus (EL-DOF) as properties desired when the interconnects are transferred onto the wafer.
In the case of a halftone phase shift mask, interference of light at its pattern-element boundary regions that is based on phase effect makes the intensity of the light at the interference-caused regions into a value of zero, so that images to be transferred can be improved in contrast. It can be expected that if the transmittance of the mask is as high as 15% or more, this phase effect becomes more remarkable so that the images to be transferred can be further improved in contrast.
In order to adjust the transmittance of a light semitransmissive layer in a halftone phase shift mask into a target range, a metal is incorporated into the light semitransmissive layer to adjust the light transmittance of the layer (Patent Literatures 1, 2, 3 and 4).
However, in the conventional halftone phase shift masks, which each have a metal-incorporated light semitransmissive layer, it is known that about the resistance of their pattern against irradiation with ArF excimer laser exposure light, and the cleaning resistance, problems are caused by the incorporation of the metal into the light semitransmissive layer. As described in Examples in Patent Literatures 1 and 2 also, Mo is frequently used as the metal to be incorporated into the light semitransmissive layer. When Mo is used, Mo is irradiated with ArF excimer laser exposure light for a long period so that water is generated from the humid atmosphere. The MoSi layer is oxidized by the generated water so that an oxide layer of silicon (Si) grows. It is known that the growth causes a problem about resistance against irradiation with the laser, this problem being a problem of causing a phenomenon that the masks are changed in pattern critical dimension. In this case, it is known that a problem about cleaning resistance is also caused, this problem being a problem that such a phenomenon is caused in the same manner in the step of cleaning the halftone phase shift masks.
In the meantime, in the case of attempting to use a metal-free light semitransmissive layer to make a halftone phase shift mask to avoid such problems about resistance against irradiation with ArF excimer laser exposure light, and cleaning resistance, regions of the mask where the light semitransmissive layer is formed become too large in transmittance (Patent Literatures 5 and 6).
As a result, for example, in Patent Literature 5, it is necessary to laminate a metal-free phase-adjusting layer (light semitransmissive layer), and a transmittance-adjusting layer which is different from the phase-adjusting layer and contains a metal onto each other, thereby forming a halftone phase shift mask since only the metal-free phase-adjusting layer (light semitransmissive layer) makes the mask too large in transmittance.
In the case of using a halftone phase shift mask usable to produce a semiconductor element to transfer a fine pattern for contact holes, lines, and others to a wafer, the following method is also known to enlarge the mask in depth of focus at the time of exposure for the transfer: a method of forming a main pattern, which is a region that is actually resolved to correspond to the fine pattern, and an assistant pattern that is not actually resolved to a wafer, using a light semitransmissive layer of the halftone phase shift mask. According to this method, a fluctuation in the critical dimension (CD) of the pattern can be decreased when the pattern is defocused since assisting diffraction light by the assistant pattern can improve exposure latitude for main pattern region.
In advanced techniques of a wafer process of transferring, onto a wafer, a mask pattern of a halftone phase shift mask used to produce a semiconductor element, in the case of transferring a fine pattern as described above for contact holes, lines or others onto the wafer, it is required in the halftone phase shift mask to form a light semitransmissive layer pattern in which the width or length of one or each pattern-element of a main pattern as described above is set, in particular, into the range of 100 to 300 nm. Furthermore, as in this case the width or length of one or each pattern-element of an assistant pattern as described above is larger, a fluctuation in the critical dimension (CD) of the pattern can be more largely decreased when the pattern is defocused. However, if the width or length is too large, an undesired pattern is resolved onto the wafer. Thus, when the width or length of the pattern-element of the main pattern is adjusted into, for example, the range of 100 to 300 nm as described above, the width or length of the pattern-element of the assistant pattern is preferably set to 60 nm or less.
Furthermore, in a wafer process of using positive tone development to transfer, onto a wafer, a mask pattern of a halftone phase shift mask used to produce a semiconductor element, in the case of forming a pattern for openings, such as contact holes or spaces, onto a resist on the wafer, a main pattern and an assistant pattern as described above are each made as a concave-element pattern in which a light semitransmissive layer in the halftone phase shift mask is partially hollowed out. By contrast, in a wafer process of using negative tone development to transfer, onto a wafer, a mask pattern of a halftone phase shift mask used to produce a semiconductor element, in the case of forming a pattern for openings, such as contact holes or spaces, onto a resist on the wafer, it is necessary that a main pattern and an assistant pattern as described above are each made as a convex-element pattern made of a light semitransmissive layer in the halftone phase shift mask. Consequently, the assistant pattern becomes a convex-element pattern made of one or more pattern-elements (each) made of the light semitransmissive layer and having a width or length of 60 nm or less.
In the use of a halftone phase shift mask used to produce an advanced semiconductor element, it is a very important theme to remove foreign matters by cleaning. In a field in which this technique is used, particularly, the following method is used as a foreign-matter-physically-removing method: a method in which when the halftone phase shift mask is cleaned, ultrasonic waves are applied to a cleaning chemical solution to use impacts based on the breaking of bubbles.
However, when the power of the ultrasonic waves is heightened to gain a strong removing power, there remains a problem that the waves give damage to a fine convex-element pattern as described, which is made of alight semitransmissive layer. It is presumed that the damage of the convex-element pattern is mainly caused by a matter that pulling-down force is applied to the convex-element pattern by impacts generated when the bubbles generated by the ultrasonic waves are broken. Accordingly, when the layer thickness of the light semitransmissive layer is large, the convex-element pattern becomes large in surface area. Thus, when bubbles are generated in the cleaning solution at the same density, a region of the convex-element pattern where the bubbles are generated becomes wide. This matter causes a problem that the damage of the convex-element pattern further increases so that the convex-element pattern is chipped. Moreover, when the layer thickness of the light semitransmissive layer is large, the convex-element pattern also suffers impacts based on bubbles generated at a higher position. In this way, the moment of the impacts becomes larger to cause a problem that the convex-element pattern is largely damaged to be chipped. It is therefore conceivable that a decrease in the layer thickness of the light semitransmissive layer is a very effective means for restraining the convex-element pattern from being damaged by ultrasonic wave cleaning.
In a method for calculating an optical proximity correction (OPC processing) for a mask pattern, approximate calculation has been mainly used. When the precision thereof needs to be made higher, an FDTD (finite-difference time-domain) method has been occasionally used, in which an exact solution is calculated. The FDTD method for calculating an exact solution is a method of developing Maxwell's equation directly to a difference equation for spatial/time regions to make a sequential computation, thereby making electric-field/magnetic-field determinations, and is a method of making a calculation, considering the layer thickness of a light semitransmissive layer pattern. In the FDTD method for calculating an exact solution, a spatial region is divided into finite elements to make a calculation on each of individual lattice points thereof. Thus, the calculation period depends on the region to be calculated. Thus, when this method is used to make a calculation for the whole of a mask, the calculation period becomes enormously long. Accordingly, for a calculation for the whole of a mask, without using any FDTD method for calculating an exact solution, an approximate calculation is used.
The approximate calculation is a simplified method which makes no consideration about the layer thickness of a light semitransmissive layer pattern as compared with the FDTD method for calculating an exact solution, and is not a method for making an exact calculation. Thus, in the case of calculating an optical proximity correction (OPC processing) for a phase shift mask used to form a fine pattern onto a wafer through an approximate calculation, this case being different from the case of making an exact solution through the FDTD method, a shielding effect caused by the layer thickness of the light semitransmissive layer pattern cannot be incorporated into calculation results. Hitherto, the thickness of a light semitransmissive layer pattern has been large so that the shielding effect has been increased, which has been caused by the layer thickness of the light semitransmissive layer pattern. As a result, conventionally, in the case of calculating an optical proximity correction (OPC processing) for a phase shift mask used to form a fine pattern onto a wafer through an approximate calculation, the shielding effect caused by the layer thickness of its light semitransmissive layer pattern is largely affected to generate a difference between results from any exact-solution calculation and ones from the approximate calculation. This matter may cause a problem that the fine pattern produced on the wafer is brought into some other member or separated by effect of the pattern to which the approximate solution calculation is used to apply an optical proximity correction (OPC processing) against design intention.
From such a matter, in making a design of a light semitransmissive layer pattern in a phase shift mask used to form a fine pattern onto a wafer, it is necessary when a calculation is made for an optical proximity correction (OPC processing) through an approximate calculation that a more intense restriction is imposed onto the design of the light semitransmissive layer pattern not to result, against design intention, in the contact or separation of the fine pattern on the wafer. Consequently, the flexibility of the design of the light semitransmissive layer pattern is low.